feat: Enable threshold ecdsa signature

[ninegua]

Turn on ECDSA signature signing by default with a test key named "Secp256k1:dfx_test_key".

Description

This enables developers to use threshold ECDSA system APIs when deploying code in a local dev environment.

How Has This Been Tested?

It has been tested manually to be working.

Checklist:

• The title of this PR complies with Conventional Commits.
• I have edited the CHANGELOG accordingly.
• I have made corresponding changes to the documentation.

[ninegua]

with a caveat that we must wait for a full DKG interfaceinterval (500 blocks in the default setting) before starting to use the key.

This will be fully addressed in https://gitlab.com/dfinity-lab/public/ic/-/merge_requests/6069. So the situation now is:

1. With this PR, users will be able to start using ECDSA key with id Secp256k1:dfx_test_key in dfx, but need to wait until a fresh dfx start has gone through the first 1000 rounds (no good indicator to find out, but roughly 1000 seconds).
2. Once MR 6069 is merged and dfx gets the the replica from a release afterwards, users will be able to use the key immediately after dfx start.